This invention relates to improved means for mixing the fuel and primary air suppy in a turbojet engine combustion chamber, particularly enlarged bowl member which enhances the burning of the fuel over a wide range of operating parameters.
Conventional combustion chambers, as a rule, are divided into two zones to assure proper stability and efficiency over a wide range of operating parameters. They typically include a primary zone fed with a portion of very rich primary air, and a dilution zone, which is fed by the remaining air flow to cool the gases issuing from the primary zone and to adjust the temperature distribution of the gases at the outlet of the combustion chamber to minimize turbine wear.
The air flow injected into the primary zone is usually a compromise between the requirements of full power operation (i.e. minimal smoke emission, chamber wall thermal strength, etc.) and the requirements during idling (i.e. flame stability and adequate efficiency). This compromise has become increasingly difficult to achieve with more modern engine combustion chambers, due to more rigorous pollution standards, the decrease in the size of the combustion chambers, and the need for the engine to run on a wider variety of fuels.
One attempt to overcome the problems presented by the aforementioned compromise has been the creation of two-module combustion chambers. One of the modules is designed for slow engine speed operation, while the other is designed for full power operation. However, this type of chamber also has its drawbacks, since it is heavy, costly to manufacture and creates regulation problems at engine speeds intermediate the idle and full power conditions.
Another attempt at solution has been to design variable-geometry injection systems in which moving baffles allow varied diaphragm action of the combustion air chamber air intakes. A substantial reduction in the combustion volume, and, hence, of the combustion chamber bulk is obtained by continuously optimizing the operation of the combustion chamber. A typical example of such a variable geometry injection system can be found in U.S. Ser. No. 792,685 to Barbier et al, filed on Oct. 29, 1985 and entitled "Variable Flow Air-Fuel Mixing Device for a Turbojet Engine" (corresponding to French Patent Application No. 2,572,463).
This type of chamber typically makes use of aerodynamic fuel injectors with intermediate bowls of the type described in U.S. Pat. No. 4,162,611 to Caruel et al. As described in this patent, the injectors are mounted on the base of the combustion chamber with an intermediate part, referred to as the bowl, extending between the fuel injector and the combustion chamber base. The intermediate bowl has a frusto-conical rim which flares outwardly in the downstream direction and is perforated by a plurality of small diameter holes through which high-speed air enters the cone of atomized fuel. The bowl enhances the fuel injection function by producing turbulence which improves the fuel atomization and its mixing with the intake air.
Research on the low-speed stability of combustion chambers with intermediate bowls, with or without variable geometry intakes, has revealed that the stability of the flame in the chamber can be improved by increasing the low-speed reactive volume of the chamber and by an improved homogenization of the fuel atomized in the primary zone. During engine idle, the flow of the injected fuel into the combustion chamber is relatively slight and the reactive volume is more confined toward the base of the combustion chamber. Therefore, the increase in the reactive volume can only take place by widening the diameter of the intermediate bowl.
However, such a widening of the diameter of the bowl increases the dwell time of the fuel within the bowl, thereby incurring the danger of self-ignition of the fuel under those operational conditions in which the pressure and temperature upstream of the combustion chamber are relatively high. This danger poses a problem for future aircraft engines in which the compression ratio will be greater than those engines currently in production. The self-ignition of the fuel may cause burns in the intermediate bowl which degrade combustion chamber operation, and require more frequent maintenance thereby increasing the engine shut-down time.